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In manufacturing integrated semiconductor circuits, many photolithography processes are performed which require repeated handling of different reticles associated with each of these processes. The reticles contain a mask of the pattern that is to be formed on the semiconductor wafer. Due to the multitude of photolithography processes, semiconductor fabrication clean rooms must store hundreds or thousands of reticles and wafers. As the most valuable component of the fabrication process, reticles are desired to have the highest degree of protection against loss, damage and contamination. If any disruptions occur in the photolithography processes, costly results follow, such as long stepper idle times, reduced productivity and missed product shipment dates. Therefore, a safe and efficient manner of storing and retrieving the reticles is desired.
Present storage and retrieval systems are designed to store and retrieve all of the reticles within a single storage unit. In conventional systems, catastrophic failures to the unit may at least temporarily disable the fabrication operations and potentially destroy the entire inventory of reticles stored therein. These conventional storage units include bays for storing the materials in a two dimensional linear matrix or grid type of arrangement. To store all of the necessary materials for the clean room manufacturing operations, a large storage unit is required. Because the height of the storage unit is limited by the ceiling of the clean room, the length of the unit must be sufficiently long to accommodate the needed storage bays. Therefore, the unit requires a large footprint which places undesirable constraints into the design and layout of the fabrication clean rooms.
A retrieval mechanism is used to retrieve the stored materials from the bays. However, because the retrieval mechanism is required to traverse great distances across the unit, difficulties arise in the repeatability of the mechanism during the retrieval process. In particular, the retrieval mechanism has difficulties in precisely traveling to each bay in the matrix. Travel imprecisions may cause the retrieval mechanism to be misaligned when the desired position in the matrix is reached which may cause damage when attempting to access the materials. As the retrieval mechanism travels to the positions at the outer edges of the grid, these travel imprecisions will be compounded and the likelihood for significant damage to the materials greatly increases. Furthermore, due to the relatively short and wide asymmetrical configuration of the units, difficulties arise in maintaining even airflow throughout these units. Specifically, because the airflow is not uniform, contaminants accumulate at the portions of the grid where the air circulation is insufficient and contaminants may be created by turbulence where the air circulation is too great. As a result, the potential damage to the stored materials increases in these areas of the unit.
A continuous flow of filtered air is desired over the stored materials to prevent particulates from accumulating and contaminating on their surfaces. One goal in the design of clean room equipment is to direct a supply of uniform filtered air over the stored materials. Typically, the flow direction of filtered air in a semiconductor clean room facility is vertical, whereby the filtered air enters through the ceiling, travels vertically downward, and then exits through a perforated floor. Equipment is preferred that utilizes airflow for controlling airborne contamination by exhausting at or near the floor to minimize the release of particles into the room so that the exposure risk of adjacent equipment to possible contamination or particulates from the discharged air is reduced. Most process equipment utilizes the natural vertical flow of air in the room as the primary source of clean filtered air by configuring the equipment with open or perforated tops and a venting system at the bottom for passing the filtered air therethrough.
If more control over the airflow quality is desired within the storage chambers of the equipment, pressurized air is often provided via ductwork and filter elements in the equipment to generate filtered air closer to the materials with xe2x80x9cpoint of usexe2x80x9d filters. More specifically, some types of equipment include a subsystem or module including fans (or blowers), and filter elements. Such subsystems, known as Fan Filter Units (FFUs) provide more control of the airflow. The fans generate positive pressure that force air through the filter element material. Many FFUs have adjustments or variable controls for the blower output, which allows control over both the pressure and the velocity of the generated air.
FFUs are typically packaged together into a module such that the blower is placed directly behind a planar filter element and enclosed in a housing that will allow the output of the fan to exhaust only through the filter element. The FFUs are then placed into the equipment either as a top mounted unit for directing airflow downward, or a side mounted for generating horizontal airflow. The ability of the blower to uniformly generate air over a large surface area filter often causes irregularities in the airflow rate exiting the filter. In systems requiring large areas of filter coverage, multiple FFUs are typically assembled into an array for generating sufficient uniformity of the discharged air.
Another goal in clean room design is to ensure that uniform airflow travels through the system after leaving the surface of the filter elements. Areas in storage chambers having non-uniform, turbulent, or little or no airflow may result from chambers with asymmetric volumes, changes to the airflow direction, multiple airflow directions, and uncontrolled venting. Some known systems incorporate FFUs and regulate the exhaust rate so that a positive internal pressure with respect to the surrounding environment is developed and maintained. As a result, contamination migration into the chamber may be reduced.
However, these systems fail to generate uniform flow of filtered air that is required within a high aspect ratio volume of storage chambers in storage and retrieval systems. Within such storage chambers, a vertical flow direction does not prevent particle accumulation on the bottom surface of the reticles or substrates stored horizontally in a shelf of the chamber. Also, airflow traveling over the edges of the reticles often causes turbulence which may lead to contamination and damage to the reticles.
Furthermore, any particulate contamination that is present on the reticles in the upper chamber may become dislodged. Accordingly, a higher concentration of particulates results in the air traveling downward through the system, and the exposure of the reticles or substrates stored in the lower storage locations are subjected to a much higher risk of contamination. Multiple units of rectangular/planar FFUs mounted along the sides of the chamber may generate an inward horizontal flow of air. However, because of the proximity of the FFUs to the movable storage locations, the air tends to coalesce at the center of the chamber after flowing past the storage locations because no efficient means exists for exhausting the air exits without generating turbulence. Such a storage chamber also would require an access point to facilitate the loading and unloading of stored reticles. At such an access point, it would not be possible to place a filter element and therefore a disruption to the uniformity of airflow would result in this area.
It is therefore desirable to have a storage and retrieval system for reticles, wafers and similar items that safely and precisely stores and retrieves the items in a clean room environment. A system is also desired which minimizes contamination by uniformly and optimally controlling the flow of air therethrough.
The present invention is directed to a storage and retrieval system for safely and efficiently storing reticles in a clean environment. An enclosed storage unit is provided for storing the reticles, and other items such as wafers and the like requiring a clean environment, which minimizes the amount of contaminants and is suitable for use in a semiconductor fabrication clean room. A retrieval unit is also provided separate from the enclosed storage unit for accessing and staging the reticles before they enter and leave the storage unit so that exposure of the storage unit is minimized. The retrieval unit includes a reticle transfer unit for passing the reticles through an access port between the storage and retrieval units.
The storage unit includes a movable storage matrix having a plurality of bays for storing the reticles. Preferably, the movable storage matrix is cylindrical with the bays located about the circumference thereof. The movable storage matrix is selectively moved or rotated by a drive mechanism that is located outside of the enclosed storage unit. The drive mechanism moves or rotates the movable storage matrix so that the access port is aligned with a desired bay or column of bays. After the drive mechanism aligns the movable storage matrix and the access port, the reticle transfer unit then retrieves the desired reticles from the corresponding bay. By rotating the movable storage matrix for accessing the reticles, the distance required by the reticle transfer unit to move is greatly reduced. Thereby, the reticles can be more precisely retrieved and stored with greater repeatability so that handling damage and contamination are minimized.
The system is designed so that the storage unit is essentially enclosed except during the storage and retrieval operations during which the storage unit is only minimally exposed. The storage unit is also designed to be substantially free of motors, moving parts, circuitry, and other contaminant generating components. For instance, features associated with the operation of the storage unit are located external to the storage unit, such as the drive mechanism for moving the movable storage matrix. By removing such components from the storage unit, these sources of contamination are reduced or even eliminated.
The compactness and symmetrical design of the system allows air to circulate uniformly throughout the storage unit. The air is vented from the storage unit to increase the uniformity of the airflow throughout the unit. By uniformly circulating and venting filtered air throughout the storage unit, the amount of potential contaminants exposed to the reticles are minimized throughout the system. The compact design also allows the system to utilize a small footprint so that greater flexibility is achieved in the placement of the system within the manufacturing room.
Other aspects, features and advantages of the present invention are disclosed in the detailed description that follows.